Sidereal time
Sidereal time is a time-keeping system. It is used by astronomers to find celestial objects. Using sidereal time it is possible to point a telescope to the proper coordinates in the night sky.
Sidereal time is a "time scale based on Earth's rate of rotation measured relative to the fixed stars".[1]
Because the Earth moves in its orbit about the Sun, a mean solar day is about four minutes longer than a sidereal day. Thus, a star appears to rise four minutes earlier each night, compared to solar time. Different stars are visible at different times of the year.
By contrast, solar time is reckoned by the movement of the Earth from the perspective of the Sun. An average solar day (24 hours) is longer than a sidereal day (23 hours, 56 minutes, 4 seconds) because of the amount the Earth moves each day in its orbit around the Sun.
Sidereal Time Media
Picture of a poster clarifying the difference between a sidereal day and the more conventional solar day
Animation showing the difference between a sidereal day and a solar day
Sidereal time vs solar time. Above left: a distant star (the small orange star) and the Sun are at culmination, on the local meridian m. Centre: only the distant star is at culmination (a mean sidereal day). Right: a few minutes later the Sun is on the local meridian again. A solar day is complete.
One of the two known surviving sidereal angle clocks in the world, made by John Arnold & Son. It was previously owned by Sir George Shuckburgh-Evelyn. It is on display in the Royal Observatory, Greenwich, London.
This astronomical clock uses dials showing both sidereal and solar time.
